Control device of variable displacement compressor

ABSTRACT

An apparatus controls a variable displacement compressor used in a refrigerant circuit. The compressor includes a drive shaft, which is rotated by an engine. When the drive shaft rotates, the compressor compresses refrigerant sent from the external refrigerant circuit and discharges the compressed refrigerant to an external refrigerant circuit. When the displacement of the compressor is minimized, the circulation of refrigerant in the refrigerant circuit is stopped. The apparatus has a control mechanism for varying the pressure in the crank chamber. A detector detects a physical quantity that reflects the heating status of the compressor. When the compressor displacement is minimized and the quantity detected by the detector indicates that the heating status of the compressor is deteriorating, a controller commands the control mechanism such that the displacement of the compressor is greater than the minimum displacement.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a control device for controllingthe displacement of a variable displacement compressor used in arefrigerant circuit of a vehicular air conditioner.

[0002] A typical variable displacement compressor used for a vehicularair conditioner is a clutchless type, or has no clutch mechanism on thepower transmission path between the compressor and a vehicle engine. Thedisplacement of the compressor is varied by changing the pressure in acrank chamber. When there is no need for cooling, for example, when anair conditioner is off, the compressor displacement is minimized.

[0003] To reduce the load applied on the engine when cooling is notdemanded, the minimum displacement of the clutchless compressor is setin the vicinity of zero. Thus, when the displacement is minimized, theflow rate of refrigerant in the refrigerant circuit is decreased. Thisreduces the amount of lubricant that flows into the compressor withrefrigerant. Hence, in the prior art, the circulation of refrigerantthrough an external refrigerant circuit is stopped when the compressordisplacement is minimized. At the same time, an internal refrigerantcircuit is formed in the compressor, and refrigerant circulates from adischarge chamber to compression chambers. Accordingly, lubricant oil inthe refrigerant lubricates the moving parts.

[0004] When the vehicle is used, for example, in winter or in the night,the air conditioner switch is likely to be off for a long time. That is,the compressor operates at the minimum displacement for a long time. Inother words, the refrigerant circulates within the compressor for a longtime. An extended period of the internal circulation of refrigerant inthe compressor excessively increases the temperature of refrigerant andlubricant, which may heats the parts of the compressor excessively.

[0005] Particularly, when carbon dioxide is used as refrigerant, therefrigerant pressure is significantly higher than a case whenchlorofluorocarbon is used as refrigerant. Therefore, the temperature ofthe interior of the compressor tends to be excessively increased whenthe compressor displacement is minimized.

SUMMARY OF THE INVENTION

[0006] Accordingly, it is an objective of the present invention toprovide a control device that improves the durability of a variabledisplacement compressor.

[0007] To achieve the foregoing and other objectives and in accordancewith the purpose of the present invention, an apparatus is provided. Theapparatus controls a variable displacement compressor used in arefrigerant circuit. The refrigerant circuit includes the compressor andan external refrigerant circuit, which is connected to the compressor.The compressor includes a drive shaft, which is rotated by an externaldrive source. When the drive shaft rotates, the compressor compressesrefrigerant sent from the external refrigerant circuit and dischargesthe compressed refrigerant to the external refrigerant circuit. Thecompressor varies the displacement of the compressor in accordance withthe pressure of a crank chamber. When the displacement of the compressoris minimized, the circulation of refrigerant in the refrigerant circuitis stopped. The apparatus comprises a control mechanism, a detector anda controller. The control mechanism varies the pressure in the crankchamber. The detector detects a physical quantity that reflects theheating status of the compressor. When the compressor displacement isminimized and the quantity detected by the detector indicates that theheating status of the compressor is deteriorating, the controllercommands the control mechanism to change the crank chamber pressure suchthat the displacement of the compressor is greater than the minimumdisplacement.

[0008] The present invention also provides a method for controlling avariable displacement compressor used in a refrigerant circuit. Therefrigerant circuit includes the compressor and an external refrigerantcircuit, which is connected to the compressor. The compressor includes adrive shaft, which is rotated by an external drive source. When thedrive shaft rotates, the compressor compresses refrigerant sent from theexternal refrigerant circuit and discharges the compressed refrigerantto the external refrigerant circuit. The compressor varies thedisplacement of the compressor in accordance with the pressure of acrank chamber. When the displacement of the compressor is minimized, thecirculation of refrigerant in the refrigerant circuit is stopped. Themethod includes detecting a physical quantity that reflects the heatingstatus of the compressor, and setting the displacement of the compressorto be greater than the minimum displacement of the compressor when thequantity detected by the detector indicates that the heating status ofthe compressor is deteriorating.

[0009] Other aspects and advantages of the invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention, together with objects and advantages thereof, maybest be understood by reference to the following description of thepresently preferred embodiments together with the accompanying drawingsin which:

[0011]FIG. 1 is a cross-sectional-view illustrating a swash plate typevariable displacement compressor according to a first embodiment of thepresent invention;

[0012]FIG. 2 is a cross-sectional view illustrating the control valve inthe compressor of FIG. 1;

[0013]FIG. 3 is a flowchart showing a procedure executed by acontroller; and

[0014]FIG. 4 is a cross-sectional view illustrating a compressoraccording to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] A control device of a swash plate type variable displacementcompressor according to a first embodiment of the present invention willnow be described with reference to FIGS. 1 to 3. The compressor is usedin a vehicular air conditioner.

[0016] (Swash Plate Type Variable Displacement Compressor)

[0017] As shown in FIG. 1, the compressor includes a cylinder block 1, afront housing member 2 connected to the front end of the cylinder block1, and a rear housing member 4 connected to the rear end of the cylinderblock 1. A valve plate assembly 3 is located between the rear housingmember 4 and the cylinder block 1. The cylinder block 1, the fronthousing member 2, and the rear housing member 4 form the housing of thecompressor.

[0018] A crank chamber 5 is defined between the cylinder block 1 and thefront housing member 2. A drive shaft 6 extends through the crankchamber 5 and is rotatably supported by the crank chamber 5. The driveshaft 6 is directly connected to an external drive source, which is-anengine E in this embodiment. There is no clutch mechanism, such as anelectromagnetic clutch, is located between the drive shaft 6 and theengine E. Thus, the drive shaft 6 is always rotated when the engine E isrunning.

[0019] Since the compressor does not have an electromagnetic clutch,which is expensive and heavy, the cost and weight of the compressor arereduced. Also, since the shock created due to activation anddeactivation of an electromagnetic clutch is eliminated, the compressorimproves the vehicle performance and response.

[0020] A lug plate 11 is fixed to the drive shaft 6 in the crank chamber5 to rotate integrally with the drive shaft 6. A drive plate, which is aswash plate 12 in this embodiment, is accommodated in the crank chamber5. The swash plate 12 slides along the drive shaft 6 and inclines withrespect to the axis of the drive shaft 6. A hinge mechanism 13 isprovided between the lug plate 11 and the swash plate 12. The hingemechanism 13 and the lug plate 11 cause the swash plate 12 to rotateintegrally with the drive shaft 6.

[0021] Cylinder bores 1 a (only one is shown in FIG. 1) are formed inthe cylinder block 1 at constant angular intervals about the axis of thedrive shaft 6. Each cylinder bore 1 a accommodates a single headedpiston 20 such that the piston 20 can reciprocate in the cylinder bore 1a. The opening of each cylinder bore 1 a is closed by the valve plateassembly 3 and the corresponding piston 20. A compression chamber 29 isdefined in each cylinder bore 1 a. The volume-of each compressionchamber 29 varies in accordance with the reciprocation of thecorresponding piston 20. The front end of each piston 20 is coupled tothe periphery of the swash plate 12 through a pair of shoes 19. Theswash plate 12 is rotated as the drive shaft 6 rotates. Rotation of theswash plate 12 is converted into reciprocation of each piston 20 by thecorresponding pair of shoes 19.

[0022] A suction chamber 21 and a discharge chamber 22 are definedbetween the valve plate assembly 3 and the rear housing member 4. Thedischarge chamber 22 is located about the suction chamber 21. The valveplate assembly 3 has suction ports 23, suction valve flaps 24, dischargeports 25, and discharge valve flaps 26. Each set of a suction port 23, asuction valve flap 24, a discharge port 25, and a discharge valve flap26 corresponds to one of the cylinder bores 1 a. Each cylinder bore 1 ais connected to the suction chamber 21 through the corresponding suctionport 23. Each cylinder bore 1 a is also connected to the dischargechamber 22 through the corresponding discharge port 25.

[0023] When each piston 20 moves from the top dead center position tothe bottom dead center position, refrigerant gas in the suction chamber21 is drawn into the corresponding cylinder bore 1 a via thecorresponding suction port 23 and suction valve flap 24. When eachpiston 20 moves from the bottom dead center position to the top deadcenter position, refrigerant gas in the corresponding cylinder bore 1 ais compressed to a predetermined pressure and is discharged to thedischarge chamber 22 via the corresponding discharge port 25 anddischarge valve flap 26.

[0024] As shown in FIG. 1, a bleed passage 27 and a supply passage 28are formed in the compressor housing. The bleed passage 27 connects thecrank chamber 5 with the suction chamber 21, which is exposed to suctionpressure Ps. The suction chamber 21 is a suction pressure zone, or a lowpressure zone. The supply passage 28 connects the discharge chamber 22,which is exposed to discharge pressure Pd, with the crank chamber 5. Thedischarge chamber 22 is a discharge pressure zone, or a high pressurezone. The supply passage 28 is regulated by a control valve CV. Thebleed passage 27, the supply passage 28, and the control valve CV formmeans for controlling the pressure in the crank chamber 5, or crankchamber pressure controlling means. The pressure in the crank chamberwill hereafter be referred to as crank chamber pressure Pc.

[0025] The opening of the control valve CV is adjusted to control theflow rate of highly pressurized gas supplied to the crank chamber 5through the supply passage 28. The crank pressure Pc is determined bythe ratio of the refrigerant gas supplied to the crank chamber 5 throughthe supply passage 28 and the flow rate of refrigerant gas conducted outfrom the crank chamber 5 through the bleed passage 27. As the crankchamber pressure Pc varies, the difference between the crank chamberpressure Pc and the pressure in the cylinder bores 1 a varies, whichchanges the inclination angle of the swash plate 12. Accordingly, thestroke of each piston 20, or the compressor displacement, is varied. Asthe control valve CV decreases the opening degree of the supply passage28, the compressor displacement is increased, and as the control valveCV increases the opening degree of the supply passage 28, the compressordisplacement is decreased.

[0026] (Refrigerant Circuit)

[0027] As shown in FIG. 1, a refrigerant circuit of the vehicular airconditioner is formed by the compressor and an external refrigerantcircuit 30. In this embodiment, carbon dioxide is used as therefrigerant. The external refrigerant circuit 30 includes a condenser31, an expansion valve 32, and an evaporator 33.

[0028] A shut-off valve 69 is located in a part of the refrigerantpassage of the external refrigerant circuit 30 between the dischargechamber 22 and the condenser 31. When the pressure at a side that isconnected to the discharge chamber 22 falls below a predetermined level,the shut-off valve 69 shuts off the refrigerant passage to stop thecirculation of refrigerant through the external refrigerant circuit 30.The shut-off valve 69 may be a differential valve that mechanicallydetects the pressure difference. Alternatively, the shut-off valve 69may be an electromagnetic valve that is controlled by a controller 70,which will be discussed below, according to a detection value of adischarge pressure sensor (not show). Further, the shut-off valve 69 maybe mechanically coupled to the swash plate 12 to shut off therefrigerant passage when the inclination angle of the swash plate 12 isminimized.

[0029] A first pressure monitoring point P1 is located in the dischargechamber 22. A second pressure monitoring point P2 is located in therefrigerant passage at a part that is spaced downstream from the firstpressure monitoring point P1 toward the condenser 31 by a predetermineddistance. The second pressure monitoring point P2 is closer to thedischarge chamber 22 than the shut-off valve 69. A fixed restrictor 68is located in the refrigerant passage between the pressure monitoringpoints P1 and P2.

[0030] As shown in FIG. 2, the control valve CV includes a valve housing45. A valve chamber 46, a communication passage 47, and a pressuresensing chamber 48 are defined in the valve housing 45. A transmissionrod 40 is extends through the valve chamber 46 and the communicationpassage 47. The transmission rod 40 moves in the axial direction, or inthe vertical direction as viewed in the drawing. The valve chamber 46 isselectively connected to and disconnected from the communication passage47 according to the position of the transmission rod 40. Thecommunication passage 47 is disconnected from the pressure sensingchamber 48 by the upper portion of the transmission rod 40.

[0031] The valve chamber 46 is connected to the discharge chamber 22through an upstream section of the supply passage 28. The communicationpassage 47 is connected to the crank chamber 5 through a downstreamsection of the supply passage 28. The valve chamber 46 and thecommunication passage 47 form a part of the supply passage 28.

[0032] A valve body 43 is formed in the axial center portion of thetransmission rod 40. The valve body 43 is located in the valve chamber46. A step defined between the valve chamber 46 and the communicationpassage 47 functions as a valve seat 53, and the communication passage47 functions as a valve hole. When the transmission rod 40 is moved fromthe position of FIG. 2, or the lowermost position, to the uppermostposition, at which the valve body 43 contacts the valve seat 53, thecommunication passage 47 is disconnected from the valve chamber 46. Thatis, the valve body 43 controls the opening degree of a control passage,which connects the crank chamber 5 to a pressure zone, in this case thesupply passage 28.

[0033] A pressure sensing member, which is a bellows 54 in thisembodiment, is located in the pressure sensing chamber 48. The upper endof the bellows 54 is fixed to the valve housing 45. The bellows 54divides the pressure sensing chamber 48 into a first pressure chamber55, which is the interior of the bellows 54, and a second pressurechamber 56, which is the exterior of the bellows 54. The upper end ofthe transmission rod 40 is fitted into the lower end of the bellows 54.

[0034] The first pressure chamber 55 is connected to the first pressuremonitoring point P1, which is the discharge chamber 22, through a firstpressure introduction passage 37. The second pressure chamber 56 isconnected to the second pressure monitoring point P2 through a secondpressure introduction passage 38. Therefore, the first pressure chamber55 is exposed to the pressure PdH monitored at the first pressuremonitoring point P1, and the second pressure chamber 56 is exposed tothe pressure PdL monitored at the second pressure monitoring point P2.In this embodiment, the pressure sensing chamber 48, the bellows 54, thefirst pressure chamber 55, and the second pressure chamber 56 form apressure sensing mechanism.

[0035] An actuator, which is a solenoid 60 in this embodiment, islocated at the lower portion of the valve housing 45. The solenoid 60includes a solenoid chamber 63, which is defined in the lowest portionof the valve housing 45. A stationary iron core 62 is located at theupper portion of the solenoid chamber 63. A movable iron core 64 islocated below the stationary core 62. The movable core 64 moves alongthe axis of the valve housing 45. A guide portion 44 of the transmissionrod 40 extends through and is slidably supported by the stationary core62. The lower end of the transmission rod 40 is fixed to the movablecore 64 in the solenoid chamber 63. A valve body urging spring 66 islocated in the solenoid chamber 63 between the stationary core 62 andthe movable core 64. The spring 66 urges the transmission rod 40 (thevalve body 43) downward as viewed in FIG. 2 through the movable core 64.

[0036] A coil 67 is wound about the stationary core 62 and the movablecore 64. The coil 67 is electrically connected to a drive circuit 71,which is controlled by a controller 70. The controller 70 is connectedto an external information detecting device 72. According to externalinformation sent from the detecting device 72, the controller 70commands the drive circuit 71 to send drive signals to the coil 67. Thecoil 67 generates an electromagnetic force that corresponds to the levelof the current from the drive circuit 71 between the movable core 64 andthe stationary core 62. The current to the coil 67 is controlled bychanging the applied voltage, thus force is applied to the pressuresensing member in accordance with external commands from the controller.Specifically, the applied voltage is controlled bypulse-width-modulation (PWM).

[0037] The external information detecting device (external informationdetector) 72 includes various sensors. The sensors of the detectingdevice 72 includes an air-conditioner switch 73 (ON/OFF switch of theair conditioner), a temperature adjuster 74 for setting a desiredtemperature in the passenger compartment, a first temperature sensor 75for detecting the temperature in the vehicle passenger compartment, asecond temperature sensor 76 for detecting the temperature of theoutside air, a third temperature sensor 77 for detecting the temperatureof the compressor housing (housing temperature), a fourth temperaturesensor 78 for detecting the temperature of fluid (refrigerant andlubricant) in the compressor, and a rotation speed sensor 79 fordetecting the speed of the output shaft of the external drive source,engine E. There is one-to-one correspondence between the speed of theengine output shaft and the speed of the compressor drive shaft 6.

[0038] In this embodiment, the second temperature sensor 76, the thirdtemperature sensor 77, the fourth temperature sensor 78, and therotation speed sensor 79 function as a detector for detecting theheating status of the compressor.

[0039] When the refrigerant temperature in the compressor increases, thehousing temperature increases. When the refrigerant temperature in thecompressor decreases, the housing temperature decreases. That is, thehousing temperature is a physical quantity that reflects the temperatureof refrigerant in the compressor. The housing temperature and therefrigerant temperature are physical quantities that reflect the heatingstatus of the compressor. Therefore, like the fourth temperature sensor78, the third temperature sensor 77 can be regarded as a refrigeranttemperature sensor that detects the temperature of fluid in thecompressor. When the external temperature increases or when the enginespeed (the speed of the drive shaft 6) increases, the refrigeranttemperature in the compressor increases. When the external temperatureor the engine speed is decreased, the refrigerant temperature isdecreased. The external temperature and the engine speed are physicalquantities that reflect the refrigerant temperature in the compressor,or the heating status of the compressor.

[0040] (Operation Characteristics of Control Valve)

[0041] The position of the transmission rod 40, or the valve opening ofthe control valve CV, is controlled in the following manner.

[0042] As shown in FIG. 2, when the coil 67 is supplied with no electriccurrent (duty ratio=0%), the position of the transmission rod 40 isdominantly determined by the downward force of the bellows 54 and thedownward force of the spring 66. Thus, the transmission rod 40 is placedat its lowermost position, and the communication passage 47 is fullyopened. The difference between the crank chamber pressure Pc and thepressure in the compression chambers 29 thus becomes great. As a result,the inclination angle of the swash plate 12 is minimized, and thedischarge displacement of the compressor is also minimized.

[0043] When the air conditioner switch 73 is off or when there is nocooling load even if the switch 73 is on (for example, when thetemperature in the passenger compartment is significantly lower than thetarget temperature), the controller 70 sets the duty ratio of currentsupplied to the coil 67 at 0%, thereby minimizing the compressordisplacement. When the compressor displacement is minimized, thepressure at one side of the shut-off valve 69 that is exposed to thepressure of the discharge chamber 22 falls below a predetermined level.This closes the shut-off valve 69 and thus stops the circulation ofrefrigerant through the external refrigerant circuit 30. Since theminimum inclination angle of the swash plate 12 is not zero, refrigerantgas is drawn into the compression chamber 29 from the suction chamber21, compressed, and discharged to the discharge chamber 22 even if thecompressor displacement is minimized.

[0044] Accordingly, an internal refrigerant circuit is formed in thecompressor. The internal circuit includes the discharge chamber 22, thesupply passage 28, the crank chamber 5, the bleed passage 27, thesuction chamber 21, and the compression chambers 29. Refrigerant,together with lubricant, circulates in the internal circuit. Thus, evenif refrigerant containing lubricant does not flow into the compressorfrom the external refrigerant circuit 30, the moving parts (for example,the swash plate 12 and the shoes 19) are reliably lubricated.

[0045] A minimum duty ratio, which is greater than 0%, is supplied tothe coil 67 of the control valve CV when, for example, there is coolingload while the air conditioner switch 73 is on. In this state, theupward electromagnetic force surpasses the resultant of the downwardforces of the bellows 54 and the spring 66, which moves the transmissionrod 40 upward. The resultant of the upward electromagnetic force and thedownward force of the spring 66 acts against the resultant of the forcesof the bellows 54 and the force based on the pressure difference ΔPd.The position of the valve body 43 of the transmission rod 40 relative tothe valve seat 53 is determined such that upward and downward forces arebalanced.

[0046] As described above, the target value of the pressure differenceΔPd is determined by the duty ratio of current supplied to the coil 67.The control valve CV automatically determines the position of thetransmission rod 40 according to changes of the pressure difference ΔPdto maintain the pressure difference ΔPd to the target value. The targetvalue of the pressure difference ΔPd is externally changed by adjustingthe duty ratio of current supplied to the coil 67.

[0047] (Characteristics of the Present Invention)

[0048] When the duty ratio of current supplied to the coil 67 is 0%, orwhen the compressor displacement is minimized and refrigerant circulateswithin the internal circuit of the compressor, the controller 70executes the routine shown in FIG. 3.

[0049] In step S101, the controller 70 determines whether the heatingstatus of the compressor is deteriorating based on physical quantitiesdetected by the temperature sensors 76-79. The heating status of thecompressor is judged to be deteriorating when any one of the followingconditions is satisfied: the external temperature is equal to or higherthan a predetermined referential value; the actual housing temperatureis equal to or higher than a predetermined referential value; the actualrefrigerant temperature is equal to or higher than a referential value;and the actual engine speed has been equal to or higher than apredetermined referential value for a predetermined period.

[0050] When the outcome of step S101 is negative, controller 70 proceedsto step S102. In step S102, the controller 70 maintains the duty ratioof current supplied to the coil 67 at 0%, which corresponds to the offstate of the air conditioner switch 73 or no cooling load. That is, thecompressor displacement is maintained at the minimum level, and therefrigerant continues circulating within the compressor.

[0051] If the outcome of step S101 is positive, that is, if the heatingstatus of the compressor is deteriorating, the controller 70 proceeds tostep S103. In step S103, the controller 70 changes the duty ratio ofcurrent supplied to the coil 67 from 0% to, for example, an intermediatevalue in the range. This procedure is referred to as protectiveprocedure in this embodiment. When the protective procedure is started,the compressor displacement is increased from the minimum displacement,which starts the circulation of refrigerant through the externalrefrigerant circuit 30. Thus, high temperature refrigerant and lubricantin the compressor are discharged from the compressor, and relativelycold refrigerant and lubricant is drawn into the compressor from theexternal refrigerant circuit 30. This prevents the temperature of theinterior of the compressor from being excessively high. In other words,the parts of the compressor are not excessively heated. Further, as thecompressor displacement is increased, lubricant is sufficiently providedto the interior of the compressor, which improves lubrication of themoving parts of the compressor.

[0052] The illustrated embodiment has the following advantages.

[0053] (1) The parts of the compressor are prevented from beingexcessively heated, and the moving parts are reliably lubricated.Accordingly, the durability of the compressor is improved. Therefore,the illustrated embodiment is advantageous when using carbon dioxiderefrigerant, the pressure of which can be significantly higher than thatof chlorofluorocarbon refrigerant, as refrigerant.

[0054] (2) The target value of the pressure difference ΔPd is determinedby the duty ratio of current supplied to the coil 67. The control valveCV automatically determines the position of the transmission rod 40according to changes of the pressure difference ΔPd to maintain thepressure difference ΔPd to the target value. The target value of thepressure difference ΔPd is externally changed by adjusting the dutyratio of current supplied to the coil 67. Since the pressure differenceΔPd represents the flow rate of refrigerant in the refrigerant circuit,the refrigerant flow rate is directly controlled by adjusting the dutyratio of current supplied to the coil 67. Therefore, if the controlvalve CV is operated at a predetermined duty ratio in the protectiveprocedure, the refrigerant flow rate in the compressor will besufficient for cooling the interior of the compressor. In other words,the cooling of the compressor is efficiently executed.

[0055] (3) The controller 70 refers to the external temperature and theengine speed when detecting the heating status of the compressor. Sincethe controller 70 detecting factors such as the external temperature andthe engine speed, which influence the heating status of the compressor,the protective procedure is executed before the heating status actuallydeteriorates. This further improves the durability of the compressor.

[0056] (4) The controller 70 executes the protective procedure when theengine speed has been equal to or higher than the referential level fora predetermined period. Therefore, the protective procedure is notexecuted when the heating status of the compressor does not deteriorate.For example, when the engine speed is higher than the referential levelonly for a short period, for example, when the vehicle is accelerated,the protective procedure is not executed. Therefore, the controlprocedure of the engine E, or the running state of the vehicle, isscarcely influenced by the protective procedure.

[0057] (5) The controller 70 refers to the housing temperature and therefrigerant temperature when detecting the heating status of thecompressor. In other words, the controller 70 directly detects theheating status of the compressor. This enables the controller 70 toappropriately execute the protective procedure.

[0058] It should be apparent to those skilled in the art that thepresent invention may be embodied in many other specific forms withoutdeparting from the spirit or scope of the invention. Particularly, itshould be understood that the invention may be embodied in the followingforms.

[0059] In the illustrated embodiment, the protective procedure isinitiated when at least one of the following conditions is satisfied:the actual external temperature is equal to or higher than thereferential external temperature; the actual housing temperature isequal to or higher than the referential housing temperature; the actualrefrigerant temperature is equal to or higher than the referentialrefrigerant temperature; and the actual engine speed has been equal toor higher than the referential speed level. Alternatively, theprotective procedure may be initiated only when two, three or all of thefour conditions are satisfied.

[0060] During the protective procedure, the duty ratio for actuating thecontrol valve CV may be varied in accordance with the degree of thedeterioration of the heating status of the compressor. For example, asthe engine speed is increased, the duty ratio may be increased forincreasing the refrigerant flow rate in the external refrigerant circuit30.

[0061]FIG. 4 illustrates a second embodiment of the present invention.In the second embodiment, the first pressure monitoring point P1 islocated in a passage in the suction pressure zone, which includes theevaporator 33 and the suction chamber 21, and the second pressuremonitoring point P2 is located at a part downstream of the firstpressure monitoring point P1 in the suction pressure zone, for example,in the suction chamber 21.

[0062] The first pressure monitoring point P1 may be located in the highpressure zone between the discharge chamber 22 and the condenser 31, andthe second pressure monitoring point P2 may be located in the crankchamber 5. The locations of the pressure monitoring points P1 and P2 arenot limited to the main circuit of the refrigerant circuit, whichincludes the evaporator 33, the suction chamber 21, the compressionchambers 29, the discharge chamber 22, and the condenser 31. Forexample, the pressure monitoring points P1, P2 may be located in anintermediate pressure zone, or a crank chamber pressure zone, in asub-circuit of the refrigerant circuit. The sub-circuit includes thesupply passage 28, the crank chamber 5, and the bleed passage 27.

[0063] The pressure sensing mechanism of the control valve CV may bechanged. For example, the control valve CV may be actuated by one of thesuction pressure Ps, the crank chamber pressure Pc, and the dischargepressure Pd. For example, in the illustrated embodiments, the secondpressure monitoring point P2 may be omitted, and the second pressurechamber 56 may be a vacuum or exposed to the atmospheric pressure.

[0064] The control valve CV may be used as a bleed control valve forcontrolling the crank chamber pressure Pc by controlling the opening ofthe bleed passage 27.

[0065] The present invention may be embodied in a control valve of awobble type variable displacement compressor.

[0066] Therefore, the present examples and embodiments are to beconsidered as illustrative and not restrictive and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalence of the appended claims.

1. An apparatus for controlling a variable displacement compressor usedin a refrigerant circuit, wherein the refrigerant circuit includes thecompressor and an external refrigerant circuit, which is connected tothe compressor, wherein the compressor includes a drive shaft, which isrotated by an external drive source, wherein, when the drive shaftrotates, the compressor compresses refrigerant sent from the externalrefrigerant circuit and discharges the compressed refrigerant to theexternal refrigerant circuit, wherein the compressor varies thedisplacement of the compressor in accordance with the pressure of acrank chamber, and wherein, when the displacement of the compressor isminimized, the circulation of refrigerant in the refrigerant circuit isstopped, the apparatus comprising: a control mechanism for varying thepressure in the crank chamber; a detector for detecting a physicalquantity that reflects the heating status of the compressor; and acontroller, wherein, when the compressor displacement is minimized andthe quantity detected by the detector indicates that the heating statusof the compressor is deteriorating, the controller commands the controlmechanism to change the crank chamber pressure such that thedisplacement of the compressor is greater than the minimum displacement.2. The apparatus according to claim 1, wherein the control mechanism hasa control passage, which connects the crank chamber to a pressure zonein which the pressure is different from the pressure of the crankchamber, and wherein the control mechanism has a control valve, whereinthe control valve controls the opening degree of the control passage,the control valve comprising: a valve body; a pressure sensing mechanismfor detecting the pressure difference between two pressure monitoringpoints which are located in the refrigerant circuit, wherein thepressure sensing mechanism has a pressure sensing member, which moves inaccordance with the pressure difference between two pressure monitoringpoints, wherein the pressure sensing member moves the valve body inaccordance with the pressure difference such that the displacement ofthe compressor is varied to counter changes of the pressure difference;and an actuator for applying force to the pressure sensing member inaccordance with external commands, wherein the force applied by theactuator corresponds to a target value of the pressure difference, andwherein the pressure sensing member moves the valve body such that thepressure difference seeks the target value.
 3. The apparatus accordingto claim 2, wherein the refrigerant circuit has a high pressure zone,which is exposed to the pressure of refrigerant that is compressed bythe compressor, and wherein the control passage is a supply passage forconnecting the crank chamber to the high pressure zone.
 4. The apparatusaccording to claim 1, wherein the physical quantity reflects the speedof the external drive source and the detector has a rotation speedsensor for detecting the quantity, and wherein, when the quantitydetected by the rotation speed sensor is greater than a predeterminedreferential value, the controller determines that the heating status ofthe compressor is deteriorating.
 5. The apparatus according to claim 4,wherein, when the quantity detected by the rotation speed sensor hasbeen greater than the predetermined referential value for more than apredetermined time period, the controller determines that the heatingstatus of the compressor is deteriorating.
 6. The apparatus according toclaim 1, wherein the physical quantity reflects the refrigeranttemperature in the compressor and the detector has a refrigeranttemperature sensor for detecting the quantity, and wherein, when thequantity detected by the refrigerant temperature sensor is greater thana referential refrigerant value, the controller determines that theheating status of the compressor is deteriorating.
 7. The apparatusaccording to claim 1, wherein the compressor is used for an airconditioner for a vehicle, and wherein the external drive source is anengine of the vehicle.
 8. The apparatus according to claim 7, whereinthe drive shaft is directly connected to the engine.
 9. The apparatusaccording to claim 1, wherein the refrigerant is carbon dioxide.
 10. Anapparatus for controlling a variable displacement compressor used in arefrigerant circuit, wherein the refrigerant circuit includes thecompressor and an external refrigerant circuit, which is connected tothe compressor, wherein the compressor includes a drive shaft, which isrotated by an external drive source, wherein, when the drive shaftrotates, the compressor compresses refrigerant sent from the externalrefrigerant circuit and discharges the compressed refrigerant to theexternal refrigerant circuit, wherein the refrigerant is carbon dioxide,wherein the compressor varies the displacement of the compressor inaccordance with the pressure of a crank chamber, and wherein, when thedisplacement of the compressor is minimized, the circulation ofrefrigerant in the refrigerant circuit is stopped, wherein thecompressor has a control passage, which connects the crank chamber to apressure zone in which the pressure is different from the pressure ofthe crank chamber, the apparatus comprising: a control mechanism forvarying the pressure in the crank chamber, and wherein the controlmechanism has a control valve, wherein the control valve controls theopening of the control passage, the control valve comprising: a valvebody; a pressure sensing mechanism for detecting the pressure differencebetween two pressure monitoring points which are located in therefrigerant circuit, wherein the pressure sensing mechanism has apressure sensing member, which moves in accordance with the pressuredifference between two pressure monitoring points, wherein the pressuresensing member moves the valve body in accordance with the pressuredifference such that the displacement of the compressor is varied tocounter changes of the pressure difference; and an actuator for applyingforce to the pressure sensing member in accordance with externalcommands; a detector for detecting a physical quantity that reflects theheating status of the compressor; and a controller, wherein thecontroller, when the compressor displacement is minimized and thequantity detected by the detector shows that the heating status of thecompressor is deteriorating, changes electric current supplied to theactuator to change the crank chamber pressure such that the displacementof the compressor is greater than the minimum displacement.
 11. Theapparatus according to claim 10, wherein the refrigerant circuit has ahigh pressure zone, which is exposed to the pressure of refrigerant thatis compressed by the compressor, and wherein the control passage is asupply passage for connecting the crank chamber to the high pressurezone.
 12. The apparatus according to claim 10, wherein the physicalquantity reflects the speed of the external drive source and thedetector has a rotation speed sensor for detecting the quantity, andwherein, when the quantity detected by the rotation speed sensor isgreater than a predetermined referential value, the controllerdetermines that the heating status of the compressor is deteriorating.13. The apparatus according to claim 12, wherein, when the quantitydetected by the rotation speed sensor is greater than the predeterminedreferential value continues more than a predetermined time period, thecontroller determines that the heating status of the compressor isdeteriorating.
 14. The apparatus according to claim 10, wherein thephysical quantity reflects the refrigerant temperature in the compressorand the detector has a refrigerant temperature sensor for detecting thequantity, and wherein, when the quantity detected by the refrigeranttemperature sensor is greater than a referential refrigerant value, thecontroller determines that the heating status of the compressor isdeteriorating.
 15. The apparatus according to claim 10, wherein thecompressor is used for an air conditioner for a vehicle, and wherein theexternal drive source is an engine of the vehicle.
 16. The apparatusaccording to claim 15, wherein the drive shaft is directly connected tothe engine.
 17. A method for controlling a variable displacementcompressor used in a refrigerant circuit, wherein the refrigerantcircuit includes the compressor and an external refrigerant circuit,which is connected to the compressor, wherein the compressor includes adrive shaft, which is rotated by an external drive source, wherein, whenthe drive shaft rotates, the compressor compresses refrigerant sent fromthe external refrigerant circuit and discharges the compressedrefrigerant to the external refrigerant circuit, wherein the compressorvaries the displacement of the compressor in accordance with thepressure of a crank chamber, and wherein, when the displacement of thecompressor is minimized, the circulation of refrigerant in therefrigerant circuit is stopped, the method including: detecting aphysical quantity that reflects the heating status of the compressor;and setting the displacement of the compressor to be greater than theminimum displacement of the compressor when the quantity detected by thedetector shows that the heating status of the compressor isdeteriorating.